I am examining various RNAV departure procedures and especially those whose purpose is to overlay the existing conventional ones. I noticed that some of them begin with a "Flight at Altitude" instruction, immediately followed by either a DF or CF leg which end at a Waypoint very close to the associated airport. As a consequence, an aircraft whose initial climb gradient is small enough will reach the specific altitude well after it has passed the aforementioned waypoint and as a result, if the CF/DF leg must be followed, it must fly backwards in order to comply.

Take, for example, ANEKI9F overlay departure procedure of Frankfurt Airport. The respective conventional procedure is charted as follows:

ANEKI9F Chart Description

Below I provide its equivalent description, as it is included in Eurocontrol's AIP library for German procedures:

Eurocontrol Charts AD 2 EDDF 5-7-34. ANEKI NINE FOXTROT:

On RWY track to 4.5 DME FFM/1.5 DME FRD or 800, whichever is later; LT (MAX IAS 210 KT until established on R356 RID), on R356 RID to RID (); on R183 RID to ANEKI ().

GPS/FMS RNAV: [A800+] - DF134 (25C)[L] / DF135 (25L)[L] - DF141 (25C) / DF142 (25L) - DF143[L] - DF137[K 210-; R] - RID[R] - ANEKI.

The problem is that fixes DF134 and DF135 are near runway's end. As a result, if the aircraft climbs even with 500ft/NM, it will still fly over those fixes. Or in other words, the aircraft will reach those fixes well before it climbs at 800ft.

Fixes in relation to the departure end of their respective runways

My question is, which of those two instructions has priority? If the aircraft reaches DF134/DF135, will the "Fly at Altitude 800ft" order be negated and the aircraft will fly at the next waypoint (DF142/DF143) or should it ignore the initial waypoints (DF134/DF135) and keep climbing until it reaches at 800ft altitude and then fly at the next waypoints (DF142/DF143)?

If someone could point me in the right direction I would highly appreciate it.

EDIT: As J. Houggard aptly stated, an assigned adverse minimum climb gradient should exist for preventing this conflict from taking place. However, no such climb gradient is assigned, according to the following table (source: Eurocontrol AIP Library).

enter image description here

Even if the table implies that the assigned climb gradient of CINDY1F procedure also applies to ANEKI9F (315ft/NM), it is still not enough.

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    $\begingroup$ Presumably the SID has a minimum climb gradient which ensures that never happens $\endgroup$ – expeditedescent Feb 19 at 20:56
  • $\begingroup$ J. Hougaard thank you for your contribution. It stands to reason for an adverse climb gradient to be specified, however if I interpret the related table correctly no such climb gradient is assigned. I will edit my question to clarify this and provide an image of the aforementioned table. $\endgroup$ – Vector Zita Feb 19 at 21:20
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    $\begingroup$ I am do not know this encoding, but 800 ft is only 436 feet above that airport, did you consider this? $\endgroup$ – Max Power Feb 20 at 0:46
  • $\begingroup$ @MaxPower you are right that given the airport's elevation, 800ft MSL altitude are not that much but the aforementioned fixes (DF134 and DF135) are so close to DER that even with 500 ft/NM, the aircraft flies over them before reaching that altitude. $\endgroup$ – Vector Zita Feb 20 at 3:45
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    $\begingroup$ Those runways are over 2 miles long, many aircraft will be at 1000ft msl by the end of the runway with a 500ft/nm climb. Even a loaded mid size 737 (5-6000ft ground roll) should clear 800msl with room to spare. $\endgroup$ – Max Power Feb 20 at 8:51

The textual description says clearly:

RWY track to 4.5 DME FFM/1.5 DME FRD or 800, whichever is later

The equivalent encoding is:

[A800+] - DF134 (25C)[L] / DF135 (25L)[L]

The meaning of this is climb to an altitude of 800ft or above, then after passing DF134/DF135 turn left.

From this EUROCONTROL document (Guidance Material for the Design of Terminal Procedures for DME/DME and GNSS Area Navigation(PDF)):

[Speed, track and altitude constraints are contained within square brackets.]

If [A Set of Constraints] is not preceded by a waypoint name, the last calculated track must be flown until the constraint is reached

The turn will thus not be initiated before both the 800 ft altitude and the waypoint are reached.

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  • $\begingroup$ Thank you DeltaLima, I appreciate your input. This means, in other words, that both conditions must be met in order for the first turn to be initiated (fly at 800 ft AND reach the waypoint, otherwise the aircraft will keep flying straight even after having passed the waypoint, until it reaches the specified altitude), have I understood this correctly? Is this the way an FMS would resolve such conflict, as it also takes a sequential series of legs as input? $\endgroup$ – Vector Zita Feb 19 at 22:19
  • $\begingroup$ To put it simply, if the specified altitude for the initial leg (for example, coded as a VA leg) is reached well after the waypoint, then the ought-to-be next leg (for example, coded as a DF leg) will be ignored? $\endgroup$ – Vector Zita Feb 19 at 22:23

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